Automatic blood pressure monitors are commonly used to periodically measure the blood pressure of a patient. In most automatic blood pressure monitors, a pressure cuff is attached to a patient's arm over the brachial artery. The cuff is first pressurized with an applied pressure that is high enough to substantially occlude the brachial artery. The cuff pressure is then gradually reduced, either continuously or in increments. As the pressure is reduced to systolic pressure, the flow of blood through the brachial artery beneath the cuff increases substantially.
When the blood flows through the brachial artery following each contraction of the heart, it imparts a pulsatile movement to the wall of the artery. This pulsatile movement is coupled to a blood pressure cuff extending over the artery as minute changes in the cuff pressure, which are known as oscillometric pulses. Automatic blood pressure monitors employing the oscillometric method measure and record the amplitude of the oscillometric pulses at a number of cuff pressures. After the blood pressure measurement had been completed, a table contains the oscillometric pulse amplitudes recorded at each cuff pressure.
In theory, the systolic, diastolic, and mean arterial blood pressures can then be determined from the values in the table using theoretical and/or empirical definitions of these parameters as a function of the amplitudes of these oscillometric pulses. However, blood pressure measurements are often adversely affected by artifact, generally produced by patient movement. Motion-induced artifact can substantially alter the measured amplitude of oscillometric pulses, thus introducing inaccuracies in the measurement of the patient's blood pressure.
Another problem associated with automatic blood pressure monitors using the oscillometric method occurs when the cuff pressure is changed from one pressure to another, particularly if the pressure is incrementally changed at a relatively high rate. The cuff pressure consists of two components, a relatively constant, or "DC", component and a relatively variable, or "AC", component. The relatively constant component defines the occlusive force of the blood pressure cuff, while the relatively variable component is produced by the oscillometric pressure pulses following each contraction of the heart.
The cuff pressure and the oscillometric pulses are typically sensed by a pressure transducer of conventional variety. The pressure transducer outputs a signal having two components which, like the pressure in the cuff itself, consist of a D.C. or relatively constant component indicative of the occlusive force, and an A.C. or relatively variable component corresponding to the oscillometric pulses imparted to the cuff. The relatively constant D.C. component can be used as an indication of occlusive force on the artery beneath the cuff, while the relatively variable A.C. component represents the oscillometric signal.
The magnitude of the relatively constant pressure is very much greater than the magnitude of the pressure changes corresponding to the oscillometric pulses. As a result, the amplitude of the D.C. component in the transducer output signal, as well as the change in the D.C. component when the cuff pressure is changed, is very much greater than the amplitude of the A.C. component in the transducer output signal. When the cuff pressure is changed, a transient A.C. component is generated that is so much larger than the normal A.C. component resulting from the oscillometric pulses that the circuits which respond to the oscillometric pulses are overloaded or are otherwise unable to process oscillometric pulses following a change in cuff pressure. A similar phenomena can occur as a result of motion artifact or from pneumatic leaks in the cuff or components in fluid communication with the cuff. As a result, oscillometric data are lost for a period of time following a cuff pressure change, thus prolonging the time needed to obtain a blood pressure measurement.